1. Field of the Invention
The present invention relates to the control of disk drives. More particularly, the present invention relates to the process of locating soft sectors using a fault tolerant method and media format for the same.
2. Art Background
Magnetic disks widely used in the computer industry have a surface on which data is encoded. The disk surface is divided into concentric circles known as tracks. Each track is divided into equal sized segments referred to as hard or servo sectors. Servo information is written at the beginning of each hard sector. This information is coherent from the inside to the outside diameter of each disk surface. A specified data pattern is written directly onto the disk which defines the start of each hard sector. The control hardware for the disk drive which reads the disk includes logic to decode this hard sector information and generate hard sector marks which are used to format data sectors. User information is stored in entities known as soft or data sectors. Data sectors are often defined as soft sectors because the data sector marks are not decoded directly from information written on the disk. Therefore, the soft sector marks used to define data fields may occur anywhere on a track.
In order to meet the needs for increased densities of data and the capacity of the drives, a method of sector division is used to increase the capacity of the drives through a recording technique known as zone density recording. As the size of each track increases with radius, more information can be stored on a track. Therefore, disks surfaces are divided into several circumferential zones. All tracks within a given zone contain a constant number of data sectors. The number of data sectors per track varies in different zones. The outermost zone contains the largest number of data sectors and the innermost zone contains the fewest. Because the number of soft sectors varies from zone to zone, data sectors have come to be known sometimes as zone sectors.
FIG. 1a illustrates the use of the term zone and track. The disk surface of FIG. 1a contains four zones, each zone containing three tracks. The number of tracks per zone is dependent upon the drive configuration and the complexity of the controlling mechanism. FIG. 1b provides an illustrative format showing the relationship between hard and soft sectors. Each hard sector is denoted by a gap followed by servo information and the user data. Gaps separate each hard sector. Soft sectors are located within the user data area of the hard sector. The signal, H sector, marks hard sector boundaries and is decoded from information recorded directly on the disk. Therefore, the location of the H sector mark cannot be moved after the manufacturing process is complete. The soft sector marks, on the other hard, are used to show the extent of a data sector and the locations of soft sector marks change from zone to zone, as the density of soft sectors per zone changes with the increase in the circumference of the track.
In an attempt to compensate for missed H sector marks (e.g., 210) a dummy soft sector count 305 is initiated. This dummy soft sector count is of a duration to estimate the approximate location of the next soft sector mark. However, problems still occur when the H sector mark is not detected. With the ability to pack in more data (soft) sectors into hard sectors, the likelihood that the data sectors are split between hard sectors is not uncommon. Thus, if a hard sector mark is missed, the ID field of the soft sector may not be detected and the soft sector counts used to delineate the soft sectors will be off and erroneous data recordings and readings will occur.
Various techniques have been used to identify the locations of soft sector. For example, in one prior method, up to ten count values are programmed by the firmware upon entering each zone. The disk drive is synchronized once for each frame; a frame defined as a group of hard sectors that has the same z-sector (soft sector) arrangement. Although this method had the benefit of limited real time firmware intervention, added data tolerance is required in the format to offset spin speed variations over the entire frame. In addition, if the hard sector mark is missed, the disk drive loses synchronization with the disk and generates soft sector signals in the wrong location causing potential loss of data. The disk drive would not be able to re-synchronize until detection of an index mark (which occurs once per revolution).
As shown in FIG. 2, in an alternate prior method, the firmware at the disk drive is loaded with count values at each hard sector location. The disk drive uses these counts to specify the location of soft sector pulses based upon H-sector pulses and previous soft sector pulses. As shown in FIG. 2, when an H sector mark is detected, a predetermined number of soft sector counts 293, 295, 300 are initiated. Although this technique decreases the tolerance the problems associated with the first method, real time firmware intervention is still required which adds complexity and overhead to the disk drive operation. Furthermore, errors still occur if a hard sector mark is missed. The present invention eliminates the problems inherent in the first method without the required firmware intervention required by the second method.